Donald Trump's personal lawyer Michael Cohen was paid at least $400,000 (£300,000) to arrange a meeting between the Ukrainian and Donald Trump, according to the BBC.
Mr Trump and Petro Poroshenko met at the White House in June. The payment was arranged by intermediaries acting for the Ukrainian leader, sources told the BBC, adding there was no suggestion that Mr Trump knew about the deal.
Mr Cohen was not registered as a “foreign agent” representing Ukraine - as required by US law.
He denies the allegations against him.
Sources in Ukraine told the BBC that Mr Cohen was brought in because Ukraine's registered lobbyists and embassy in Washington DC could get their president little more than a brief photo-op with Mr Trump, and Mr Poroshenko needed something that could be portrayed as "talks".
The Ukrainian politician, who is not named by the BBC, used his own contacts at a Jewish charity in New York state, Chabad of Port Washington, which eventually led to Mr Cohen. Both the former aide and the politician have denied the allegations.
Mr Cohen agreed to assist, the BBC reported, but right up until the last minute it was all hanging in the balance; the White House schedule said only that Mr Poroshenko would "drop in" to the Oval Office while Mr Trump was having staff meetings.
The Ukrainian side were angry, the official told the BBC, because Mr Cohen had taken "hundreds of thousands" of dollars from them for something it seemed he could not deliver.
"Poroshenko's inner circle were shocked by how dirty this whole arrangement [with Cohen] was," a source said.
The meeting did finally happen, however, and, with the Ukrainian leader sitting by his side, Mr Trump said it was a “great honour” to meet Mr Poroshenko and that “a lot of progress has been made” in the US relationship with Ukraine.
Returning to Ukraine, Mr Poroshenko, the billionaire pro-Western leader elected in 2014, was reportedly satisfied that he had met Mr Trump before Vladimir Putin, the Russian president.
Shortly after, his Ukraine's anti-corruption agency stopped its investigation into Mr Trump's former campaign manager, Paul Manafort, who was accused of not declaring his ties to a pro-Putin group in Ukraine.
A source in Ukraine told the BBC that Mr Manafort, who denies all allegations against him, had actually received many more millions from Ukraine that the Americans did not know about.
Scheduled visits by Mr Poroshenko's senior aide to see Robert Mueller, leading the investigation into Russian meddling in the US election, and the CIA director, in November and December, were cancelled.
One source in Kiev said Mr Poroshenko had given Mr Trump "a gift" - making sure that Ukraine would find no more evidence to give the US inquiry into whether the Trump campaign "colluded" with Russia.
The BBC’s sources in Kiev appear to be supported by research from Michael Avenatti, the lawyer for porn actress Stormy Daniels.
Mr Avenatti, who claimed that Daniels was paid $130,000 in “hush money” by Mr Cohen, reported that Suspicious Activity Reports filed by Mr Cohen's bank to the US Treasury showed he had received money from "Ukrainian interests".
A spokesperson for the president's office in Ukraine told The Telegraph that the meeting was arranged through the Ukrainian embassy in Washington "exclusively by means of official diplomatic channels".
The spokesperson claimed they were not given the opportunity to comment by the BBC over the story and threatened to sue the broadcaster, adding: "We believe that the blatant disinformation that has been disseminated is a part of a fake campaign aimed at discrediting Ukraine-US relations, as well as a personal attack against the Presidents of Ukraine and the US."